Scale and definition challenges in quantitative hydrogeology

Many basic hydrogeology definitions are relative and influenced by anthropocentrism. The aquifer definition is an obvious example. Spatial and time scales influence the values of the main parameters/properties to be used for answering in a quantified way any practical question.
For example, porosity describes the ratio between the volume of pores, cracks, and fissures and the total volume of a studied geological medium. This notion implies a volume averaging of the medium characteristics using the concept of Representative Elementary Volume (REV). Small volumes can contain only pores while larger volumes typically contain both pores and fissures. Porosity can be highly scale-dependent, and different porosity values can be measured for the same geological formation. Furthermore, groundwater in the pores and cracks can be partly immobile or mobile. So, the porosity actively involved in groundwater flow can be discussed. A ‘mobile water porosity’ can be defined, but this remains highly dependent on the existing pressure conditions in the geological medium. In unconfined conditions, the term ‘effective porosity’ corresponds usually to the drainage porosity corresponding also to the specific yield or the storage coefficient.
When dealing with solute transport and remediation of contaminated sites, another ‘effective porosity’ is needed to describe the advection velocity of the contaminant. This ‘mobile water porosity’ acting in solute transport processes takes typically lower values than the ‘effective porosity’ of drainage.